1. Field of the Invention
This invention relates generally to the formation of deformable, optically anisotropic pitches particularly useful in the formation of shaped carbon articles, such as electrodes and the like. More particularly, this invention relates to the formation of deformable, optically anisotropic pitches particularly useful in the formation of carbon and graphite filaments of continuous lengths.
2. Description of the Prior Art
Petroleum, coal tar and chemical pitches because of their high carbon to hydrogen ratio, have the potential, at least, to be used commercially in forming a wide variety of carbon artifacts. One carbon artifact of particular commercial interest today is carbon fiber. Hence, although particular reference is made herein to carbon fiber technology, it will be appreciated that this invention has applicability in areas other than carbon fiber formation.
Referring now in particular to carbon fibers, suffice it to say that the use of carbon fibers in reinforcing plastic and metal matrices has gained considerable commercial acceptance where the exceptional properties of the reinforced composite materials, such as their high strength to weight ratios, clearly offset the generally high costs associated with preparing them. It is generally accepted that large scale use of carbon fibers as a reinforcing material would gain even greater acceptance in the marketplace if the costs associated with the formation of the fibers could be substantially reduced. Much of the commercially available carbon fiber today is obtained by carbonizing synthetic polymers, such as polyacrylonitrile. The high cost of such carbon fibers is due in part to the high cost of the polyacrylonitrile fiber being carbonized, the low yield of carbon fiber resulting therefrom and the processing steps necessary to maintain a desirable physical structure of the atoms in the fiber which will impart adequate strength to the resultant carbon fiber.
More recently, the formation of carbon fibers from relatively inexpensive pitches has received considerable attention. Use of relatively inexpensive pitch materials, however, has not substantially reduced the cost of the formation of carbon fibers having commercially acceptable physical properties.
To date, all high strength, high modulus carbon fibers prepared from pitches are characterized, in part, by the presence of carbon crystallites preferentially aligned parallel to the fiber axis. This highly oriented type of structure in the carbon fiber has been obtained either by introducing orientation into the precursor pitch fiber by high temperature stretching of the pitch fiber or by first forming a pitch for fiber formation which possesses considerable structure.
High temperature stretching of pitch fibers has not resulted in inexpensive fibers of adequate strength and modulus for numerous reasons including the difficulty in stretching the pitch fiber at high temperatures without breaking the fibers, and the concomitant cost of equipment for carrying out the stretching operation, to mention a few.
In forming a carbon fiber from a pitch material which has a high degree of orientation, it has been considered necessary to thermally transform the carbonaceous pitch, at least in part, to a liquid crystal or the so-called "mesophase" state. This mesophase state has been characterized as consisting of two components, one of which is an optically anisotropic, highly oriented material having a pseudocrystalline nature and the other, an isotropic nonoriented material. As is disclosed, for example, in U.S. Pat. No. 4,005,187, the nonmesophase portion of the pitch is readily soluble in pyridine and quinoline and the mesophase portion is insoluble in these solvents. Indeed, the amount of insoluble material in the thermally treated pitch is treated as being equivalent to the amount of mesophase formed. In any event, this thermal processing step is expensive, particularly in terms of mesophase production rate. For example, at 350.degree. C., the minimum temperature generally required to convert an isotropic pitch to the mesophase state, at least one week of heating is usually necessary and then mesophase content of the pitch is only about 40%. In addition thereto, the formation of fibers from pitches containing as much as 60% of mesophase material, for example, still requires extensive and costly postspinning treatments in order to provide a fiber which has the requisite Young's modulus rendering these fibers commercially attractive and important.